1. Technical Field
The present invention relates to a cleaning solution for use in removing a damaged portion of a ferroelectric layer, and a method for removing a damaged portion of a ferroelectric layer using the cleaning solution.
2. Discussion of Related Art
A ferroelectric random access memory (FRAM) can retain its contents without power for years. FRAM capacitors are kept in a charged state while the supply of power is suspended. During a patterning process for forming a FRAM capacitor, active components of etching gases used for the patterning, such as argon (Ar), fluorine (F) or chlorine (Cl), react with electrode materials or ferroelectric materials, thereby resulting in byproducts such as PbO2, ZrO2, TiO2 or TiO3, on the surface of a ferroelectric layer. As a result, the ferroelectric layer is damaged by the byproducts. The damaged ferroelectric layer serves as a leakage current path between upper and lower electrodes, thereby degrading the electrical properties of the FRAM capacitor.
In the manufacture of the FRAM capacitor, annealing is carried out so as to provide the FRAM capacitor with a stable perovskite structure. However, even after the annealing is completed, a pyrochlore structure, which is a semi-stable phase, appears from the top of the ferroelectric layer to a predetermined depth. If an upper electrode layer is deposited over the ferroelectric layer having the pyrochlore structure, a desirable FRAM capacitor having superior performance cannot be obtained. In this aspect, the ferroelectric layer having the pyrochlore structure is regarded as a kind of damaged layer, relative to the whole ferroelectric layer with the perovskite structure.
Thus, there is a need to perform a cleaning step for removing such a ferroelectric layer having the semi-stable pyrochlore structure and a portion of the ferroelectric layer that is damaged during an etching process in the manufacture of the FRAM capacitor. A conventional cleaning solution used in the cleaning step includes hydrogen fluoride (HF) diluted with an alcoholic solvent, such as methanol, ethanol or isopropyl alcohol, in a predetermined ratio. However, because the alcoholic solvent of the cleaning solution is highly volatile, the alcoholic solvent evaporates with time, hence the cleaning (etching) capability of the cleaning solution cannot be kept constant, thereby shortening the lifetime of the cleaning solution. Another disadvantage in using the conventional cleaning solution lies in that after the cleaning is completed, a further rinsing step with isopropyl alcohol (IPA) is needed before a rinsing step with deionized water. This is because the rinsing step with deionized water is insufficient to completely remove the cleaning solution from the surface of the substrate, which causes corrosion of the substrate.
Therefore, a need exists for providing a cleaning solution capable of selectively removing a damaged portion of a ferroelectric layer, and has a consistent cleaning capability for a longer period of time.
A method is also needed for selectively removing a damaged portion of a ferroelectric layer using the cleaning solution.